TUESDAY, April 20 (HealthDay News) -- Although pregnant women make up about 1% of the U.S. population at any given time, during the 2009 H1N1 swine flu outbreak they made up 5% of deaths, a new study has found.

Researchers also found that pregnant women who waited four days before seeking treatment were admitted to the intensive care unit (ICU) nearly 57% of the time, compared to only about 9% for pregnant women who didn't delay seeking treatment.

The H1N1-linked death rate among pregnant women was "more than we would have expected," said one of the study's authors, Dr. Sonja Rasmussen, a medical officer with the U.S. Centers for Disease Control and Prevention. "We also learned that early treatment with anti-viral medications lowers the chances of being admitted to the ICU, and lowers the chances of death."

The Latest on H1N1 Swine Flu

Results of the study were published in the April 21 issue of the Journal of the American Medical Association.

H1N1 flu was first reported in the United States on April 21, 2009, by officials at the CDC. Two children in California had been infected with the new virus. The virus rapidly spread worldwide and was declared a pandemic in June, according to background information in the study.

Because pregnant women tend to be more vulnerable in any flu outbreak, the CDC asked local and state health departments to report any confirmed or probably case of H1N1 in a pregnant woman from April 2009 through Dec. 31, 2009.

They received reports on 788 pregnant women with H1N1 symptoms during that time period. Thirty of the women died from complications of H1N1, which represents 5% of all H1N1 deaths, according to the study.

In total, 509 of the women were hospitalized, and of those, 115 were admitted to the intensive care unit.

The study found that when pregnant women sought early treatment -- within two days of symptom onset -- they had less than a 10% chance of being admitted to the ICU, while women who waited more than four days for treatment had almost a 60% risk of ending up in intensive care.

Only one death occurred among pregnant women who sought early treatment, the study found.

The researchers also found that more deaths -- more than 64% -- occurred in the third trimester of pregnancy, compared to the second trimester (almost 27%) or the first trimester (about 7%).

Dr. John Carpenter, division director of infectious disease at Scott & White Healthcare in Temple, Texas, said part of the reason why women may have more trouble with flu as they get farther along in their pregnancies is that the uterus is pushing on the diaphragm, making breathing a little bit harder, and making it more difficult to push out a good cough.

"While you're pregnant, be especially alert for respiratory illnesses," advised Carpenter. He said that antiviral medications, such as Tamiflu, can be given during pregnancy.

Rasmussen agreed that drugs such as Tamiflu can be given during pregnancy, and there are no known serious risks to mother or baby from these medications.

The bottom line, said Rasmussen, is that "early treatment makes a difference in complications and mortality."

Carpenter also advised getting a flu shot before getting pregnant and to avoid people with respiratory illnesses during your pregnancy.

While Rasmussen said the CDC can't predict whether the H1N1 virus will be a big threat again, it is already being included in this year's seasonal flu vaccine. And, she said, if women haven't gotten a flu vaccine prior to pregnancy, they can get one at any time during their pregnancy.